This invention relates generally to thermal papers, and, more particularly, to thermal papers whose imaging characteristics are enhanced by the application of a tough, coarse surfaced heat-metering layer. This invention further relates to methods of making enhanced thermal papers having a tough, coarse surfaced heat-metering layer.
Thermal papers are used in a wide variety of thermal printers including facsimile machines, calculators, portable terminals, label printers, plotters, tag and ticket printers, printers for use with personal computers and other hard copy devices. Thermal imaging is widely used because it is economical, quiet, clean, and reliable.
Thermal papers generally comprise a substrate layer and a heat-sensitive recording layer which is applied to the substrate. The substrate is usually paper, although tag stock, pressure sensitive labels, laminated papers and backings, as well as synthetic substrates may be used.
A variety of different chemistries are used to produce images in the heat-sensitive recording layer. Generally, these chemistries include a dye system which changes color or "develops" on the application of heat, and a binding agent. A typical dye system, described for example in U.S. Pat. No. 5,256,619, uses a combination of a leuco dye and an acidic co-reactant.
Typically, images are formed in the heat-sensitive recording layer using a thermal printhead in which a matrix of point sources of heat are turned on and off to form the images as the thermal paper moves past the thermal printhead.
One of the most common uses of thermal papers is in machines for receiving facsimile transmissions. The thermal paper used with such facsimile machines is typically made as inexpensively as possible, using a thin paper substrate formed from mechanical pulps and as little of an inexpensive chemistry as possible in the heat-sensitive recording layer. The paper substrate typically will not be sized or calendared and therefore will have both voids in the paper surface and paper fibers which protrude into the heat-sensitive recording layer. Additionally, facsimile papers include a lubricant such as zinc stearate in the heat-sensitive recording layer. Facsimile papers have no topcoat, a feature found in more expensive, higher quality thermal papers, as discussed below.
Chart papers are thermal papers used in thermal plotters. They differ from the thermal papers used in facsimile machines primarily in that certain additives are present in the heat-sensitive recording layer to make that layer more durable. Like thermal papers used with facsimile machines, chart papers typically are not topcoated.
The image quality or definition obtained using facsimile paper is generally poor for various reasons, including those explained below, but it is considered acceptable since it is "person-readable". For example, portions of the image are lost where the heat-sensitive recording layer is disrupted by paper fibers protruding from the paper substrate or where the recording layer has voids or depressions due either to its thinness or to underlying voids in the paper substrate. Additionally, surface coarseness has been thought to interfere with heat transfer from the printhead, further impairing image quality.
Border areas of facsimile paper images often are irregular and extend beyond the image presented to the thermal printhead. This problem, which is known as "bloom", comes about due to heat flow into the heat-sensitive recording layer beyond the borders of the image presented by the heat point sources in the thermal printhead. The images produced with facsimile paper thus are inferior to those obtained with press or laser printing in which the printed images are sharply defined and correspond precisely to the images intended to be produced.
In addition to the undesirable bloom and limited image definition obtained with facsimile paper, such paper is also highly vulnerable to scuffing, erasures, and development of non-image areas upon exposure to organic solvents, water, oils, plasticizers, and other materials. Probably the most widely recognized problem with facsimile papers are the fingerprints arising from contact with the sebum of the fingers of the person handling the facsimile paper.
Thermal papers of significantly better quality than facsimile paper are available. In these papers, thicker, calendared and sized paper substrates are used in order to improve imaging characteristics as well as resistance to the above noted incompatible materials. In these papers, for example, the voids in the substrate surface which cause depressions in the heat-sensitive recording layer as well as the paper fibers which protrude into and disrupt the heat-sensitive recording layer typically will be minimized or eliminated. Additionally, in such improved thermal papers, the heat-sensitive recording layer may be made thicker and may employ more sophisticated chemistries which reduce undesirable bloom.
In such better quality thermal papers, protective topcoatings are applied over the heat-sensitive recording top layer in order to minimize or eliminate problems arising from exposure to solvents, water, light, plasticizers, and human sebum. Typically, the surfaces of these protective topcoatings are made as smooth and glossy as possible because such smooth and glossy surfaces have been thought to enhance the final image quality by improving heat transfer while minimizing friction as the paper goes past the thermal printhead. These protective topcoatings, however, tend to soften when heated.
As might be expected, higher quality topcoated thermal papers made with thicker, sized and highly calendared substrates, with smooth protective coatings and with thicker and more advanced heat-sensitive recording layer chemistries (referred to below as "topcoated thermal papers") are far more expensive than facsimile paper. Nevertheless, such topcoated thermal papers are widely used because facsimile papers have in the past been considered inadequate for many thermal printing applications.
Topcoated thermal papers are used in a wide variety of non-facsimile thermal printing applications such as in making tags, labels and tickets which bear alphanumeric symbols as well as bar codes or other machine-readable indicia. Often, thermally printed tags, labels and tickets are made by laminating topcoated thermal paper to a thicker cellulosic substrate by known processes. One such process is described in Goncalves, U.S. Pat. No. 5,092,949, which is directed to a laminated, thermally imageable baggage tag having a bottom layer, reinforcing fibers and a smooth topcoated thermal paper top sheet.
Although it has been thought that facsimile paper could be used only in applications in which relatively imprecise images are acceptable, such as in receiving facsimile transmissions, in plotting charts and graphs, and in calculators, it would be highly desirable to find a way in which facsimile paper could be simply and inexpensively enhanced to enable it to produce precise images on a par with current topcoated thermal papers. If such more precise images were obtainable, facsimile paper could be used in other thermal printing applications, including in making tags, labels and tickets which are to be printed with bar codes or other machine-readable indicia requiring a high degree of precision.
It is therefore an object of the present invention to provide a coating for facsimile paper which meters and directs the flow of heat from the heat point sources in a thermal printhead to the heat-sensitive recording layer of the paper in such a way that undesirable bloom is reduced or eliminated and an image of sufficient definition for bar code and other machine-readable applications can be produced.
These and other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying specification, claims and drawings.